Over the past few days I have noticed by betta getting a little bloated just behind his gills. I have no idea why but I gave the tank a 50% water change just a few moments ago just in case, aswell as checked the Ph. Nitrate ect. which were all normal. Could it be overfeeding? I have noticed over a bit of a time span in which I have increased his food intake a bit (advice from the local petstoe). Can any please tell me what this is or could be?

Overfeeding is a distinct possibility, which could in turn lead to constipation and SBD. No need to increase the amount you feed. I feed bettas very little food (like 1 pellet/flake) very frequently (like 5 times per day). This helps to avoid them becoming "stuffed".